全文获取类型
收费全文 | 533篇 |
免费 | 31篇 |
国内免费 | 5篇 |
专业分类
化学 | 262篇 |
晶体学 | 2篇 |
力学 | 56篇 |
数学 | 98篇 |
物理学 | 151篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 12篇 |
2021年 | 27篇 |
2020年 | 25篇 |
2019年 | 23篇 |
2018年 | 42篇 |
2017年 | 30篇 |
2016年 | 47篇 |
2015年 | 40篇 |
2014年 | 24篇 |
2013年 | 72篇 |
2012年 | 37篇 |
2011年 | 48篇 |
2010年 | 28篇 |
2009年 | 31篇 |
2008年 | 19篇 |
2007年 | 14篇 |
2006年 | 13篇 |
2005年 | 9篇 |
2004年 | 1篇 |
2003年 | 7篇 |
2002年 | 4篇 |
2001年 | 1篇 |
1999年 | 3篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1989年 | 1篇 |
1984年 | 2篇 |
排序方式: 共有569条查询结果,搜索用时 21 毫秒
161.
Safa Raziyeh Soltani Goharrizi Ataallah Jafari Saeed Jahanshahi Javaran Ebrahim 《Journal of Thermal Analysis and Calorimetry》2021,144(6):2497-2514
Journal of Thermal Analysis and Calorimetry - In the present study, double-diffusive mixed convection related to the heat and mass transfer of the solid particles dissolution in a shear flow was... 相似文献
162.
Diels–Alder reactions of 3-vinylindoles were carried out with methyl acrylate and N-phenylmaleimide as dienophiles under microwave conditions to furnish unreported tetrahydrocarbazoles regio- and stereoselectively in good yields. Further dehydrogenation resulted in new substituted carbazoles. 相似文献
163.
Differential pulse polarography was used for simultaneous determination of Sn2+ and Pb2+. But there is a problem for simultaneous determination and it is high overlapped DPPs of mentioned cations that their determination is impossible in the presence of each other, so multivariate calibration methods as chemomatrics methods were used for this determination. There are some disadvantageous for multivariate calibration methods that can be solved by a new and simple method called net analyte signal standard addition method. This method has some advantages, such as: the use of a full voltammogram, realization in a single step, therefore it does not require calibration and prediction steps and only a few measurements are required for the determination. 相似文献
164.
Iman Jafari Mohamadreza Shakiba Fatemeh Khosravi Seeram Ramakrishna Ehsan Abasi Ying Shen Teo Mohammadreza Kalaee Majid Abdouss Ahmad Ramazani S. A Omid Moradi Erfan Rezvani Ghomi 《Molecules (Basel, Switzerland)》2021,26(6)
The incorporation of nanofillers such as graphene into polymers has shown significant improvements in mechanical characteristics, thermal stability, and conductivity of resulting polymeric nanocomposites. To this aim, the influence of incorporation of graphene nanosheets into ultra-high molecular weight polyethylene (UHMWPE) on the thermal behavior and degradation kinetics of UHMWPE/graphene nanocomposites was investigated. Scanning electron microscopy (SEM) analysis revealed that graphene nanosheets were uniformly spread throughout the UHMWPE’s molecular chains. X-Ray Diffraction (XRD) data posited that the morphology of dispersed graphene sheets in UHMWPE was exfoliated. Non-isothermal differential scanning calorimetry (DSC) studies identified a more pronounced increase in melting temperatures and latent heat of fusions in nanocomposites compared to UHMWPE at lower concentrations of graphene. Thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) revealed that UHMWPE’s thermal stability has been improved via incorporating graphene nanosheets. Further, degradation kinetics of neat polymer and nanocomposites have been modeled using equations such as Friedman, Ozawa–Flynn–Wall (OFW), Kissinger, and Augis and Bennett’s. The "Model-Fitting Method” showed that the auto-catalytic nth-order mechanism provided a highly consistent and appropriate fit to describe the degradation mechanism of UHMWPE and its graphene nanocomposites. In addition, the calculated activation energy (Ea) of thermal degradation was enhanced by an increase in graphene concentration up to 2.1 wt.%, followed by a decrease in higher graphene content. 相似文献
165.
Ali Nakhaei Pour Yahya Zamani Kheirolah Jafari Jozani Jafar Yeganeh Mehr 《Reaction Kinetics and Catalysis Letters》2005,86(1):157-162
Summary The effect of La2O3 and TiO2 on product selectivity, methane conversion and coke formation over NiO/MgO/ α -Al2O3 catalyst were studied in a simultaneous steam and CO2 reforming of methane to syngas. La2O3 and TiO2 were added to the catalyst via incipient wetness impregnation and bulk precipitation techniques and catalyst activity was tested in a fixed bed quartz reactor. Results reveal that although the addition of these oxides has no effect on the product selectivity and methane conversion, but can reduce coke formation on the surface of the catalysts as it can enhance the mobility of lattice oxygen anions. The results further show that the catalysts prepared by bulk precipitation technique decrease the coke formation more effectively. 相似文献
166.
Masumeh Foroutan Nosrat Heidari Maryam Mohammadlou Amin Jafari Sojahrood 《The Journal of chemical thermodynamics》2009,41(2):227-231
(Liquid + liquid) equilibrium (LLE) data of quaternary aqueous system containing polyoxyethylene (20) cetyl ether (with abbreviation name Brij 58, non-ionic surfactant), diammonium hydrogen phosphate, and poly ethylene glycol (PEG) with three molar masses {MW = (1000, 6000, and 35,000) g · mol?1} have been determined experimentally at T = 313.15 K.Furthermore, the Flory–Huggins theory with two electrostatic terms (Debye–Hückel and Pitzer–Debye–Hückel equations) have been used to calculate the phase behavior of the quaternary systems and (surfactant + polymer) interaction parameter as well as interaction parameters between other species. Temperature dependency of the parameters of the Flory–Huggins theory has been obtained.Also an effort have been done to show that addition of PEG as well as increasing the temperature can shift the binodal curves of the ternary aqueous system containing surfactant and salt to lower mole fraction of salt. Also the effect of polymer molar mass on the binodal diagram displacement has been discussed. 相似文献
167.
Seyed Heidar Mirjahanmardi Faramarz Afshar Taromi Roya Zahedi Mehdi Nekoomanesh Haghighi Roghayeh Jamjah Gholamali Jafari Asl 《Polymer Science Series B》2016,58(6):619-628
The multi-ether compounds with different numbers of methoxy groups containing 1,3-dimethoxy-2,2-bis(methoxymethyl)propane and 1-methoxy-2,2-bis(methoxymethyl)butane were synthesized using the Williamson reaction from pentaerythritol and 1,1,1-tris(hydroxymethyl)propane, respectively, in the presence of sodium hydride and methyl iodide in tetrahydrofuran and they were characterized by 1H NMR, 13C NMR, and FTIR spectroscopy. These compounds were employed as external donors in the polymerization of propylene using the industrial Ziegler-Natta catalyst. A commercial spherical MgCl2-supported Ziegler-Natta catalyst containing diisobutyl phthalate as the internal donor was used for the polymerization of propylene. The role of ether compounds and industrial alkoxysilanes on the properties of polypropylene were studied using the xylene solubility method, melt flow index, gel permeation chromatography, scanning electron microscopy, and differential scanning calorimetry. The addition of the electron donors has led to improvements in the activity and selectivity of the Ziegler-Natta catalyst system. 相似文献
168.
Vocal perturbation, harmonics-to-noise, and intensity measures were obtained for 10 subjects during three experimental tasks: (a) prolonged /a/, (b) /pa/ with vowel prolonged, and (c) same as (b) with subjects wearing a pneumotachographic mask and oral pressure tube inserted between the lips. There were no statistically significant differences among the experimental conditions for any of the measures. The findings suggest that a single task may be used to obtain airflow, oral pressure, and acoustic measures of vocal performance. Observed differences in jitter and harmonic-to-noise means for the male and female speakers are discussed. 相似文献
169.
Bounds on the 2-Rainbow Domination Number of Graphs 总被引:1,自引:0,他引:1
A 2-rainbow domination function of a graph G is a function f that assigns to each vertex a set of colors chosen from the set {1, 2}, such that for any ${v\in V(G), f(v)=\emptyset}$ implies ${\bigcup_{u\in N(v)}f(u)=\{1,2\}.}$ The 2-rainbow domination number γ r2(G) of a graph G is the minimum ${w(f)=\Sigma_{v\in V}|f(v)|}$ over all such functions f. Let G be a connected graph of order |V(G)| = n ≥ 3. We prove that γ r2(G) ≤ 3n/4 and we characterize the graphs achieving equality. We also prove a lower bound for 2-rainbow domination number of a tree using its domination number. Some other lower and upper bounds of γ r2(G) in terms of diameter are also given. 相似文献
170.
M. Hafez S. A. Jafari 《The European Physical Journal B - Condensed Matter and Complex Systems》2010,78(3):323-333
We use perturbative continuous unitary transformations (PCUT) to study
the one dimensional extended ionic Hubbard model (EIHM) at half-filling in the band insulator
region. The extended ionic Hubbard model, in addition to the usual ionic
Hubbard model, includes an inter-site nearest-neighbor (n.n.) repulsion, V. We consider
the ionic potential as unperturbed part of the Hamiltonian, while the hopping and interaction
(quartic) terms are treated as perturbation.
We calculate total energy and ionicity in the ground state.
Above the ground state, (i) we calculate the single particle excitation spectrum by adding
an electron or a hole to the system; (ii) the coherence-length
and spectrum of electron-hole excitation are obtained.
Our calculations reveal that for V = 0, there are two triplet bound state modes and three
singlet modes, two anti-bound states and one bound state, while for finite values
of V there are four excitonic bound states corresponding to two singlet and two triplet modes.
The major role of on-site Coulomb repulsion U is to split singlet and triplet
collective excitation branches, while V tends to pull the singlet branches below
the continuum to make them bound states. 相似文献